Question: Which of these numbers is prime? ${27,\ 49,\ 67,\ 81,\ 92}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 27 are 1, 3, 9, and 27. The factors of 49 are 1, 7, and 49. The factors of 67 are 1 and 67. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 92 are 1, 2, 4, 23, 46, and 92. Thus, 67 is a prime number.